Not too sure if my advise will work for extension hair but you can do a hair dye removal technique called a "Bleach bath" or "soap cap" Mix equal parts shampoo, water and hair bleach to a creamy consistancy (like yogurt) then apply to the hair. This stuff works quickly so it'll be best to keep an eye on it and scrape of the mixture every two minutes to see how much dye has been lifted out. Don't leave it on for more than 15 minutes. Rinse well, use a reconstructor, followed by a deep moisturizing condtioner. Wait about 24 hours to recolor with any dye that requires a developer.